The general purpose of the invention is to allow a precise measurement of phase shift between two tone burst signals initially derived from the same periodic source but delayed by different amounts through either path length differences, propagation velocity differences, or both. For example, the invention can be used to accurately measure the phase shift encountered by an acoustic wave striking the front surface and back surface of a sample. With velocity known, the resulting measurement would be of sample thickness. If thickness is known, the resulting measurement is of sound velocity. Both the above use the relationship Fm=mV/2l where Fm is the harmonic frequency, m the harmonic number (an integer), V the propagation velocity, an l the sample thickness along the propagation direction. The measured parameters are the Fm and the m value. The results apply to all sources of radiation and are not limited to acoustics.
Prior art can be described primarily as pulse timing technology. In general, a delta function or step function pulse of energy is emitted by a source and reflected by a target. This type of measurement is in the time domain. The initial pulse time, .tau..sub.o is used to start a counter and the first reflection, at time .tau..sub.i is used to stop a counter. .tau..sub.i -.tau..sub.o is .DELTA..tau., the travel time. Sometimes, a second reflector is used at time .tau..sub.2 and the measurement consists of .tau..sub.2 -.tau..sub.1 =.DELTA..tau.'. A second type of measurement system exists in the frequency domain. A continuous wave (CW) or pulsed CW source is used to generate a wave. The CW case requires a good sample "geometry" so that standing waves exist. The frequency of resonance is given by Fm=mV/2l while the frequency difference between harmonic is just .DELTA.F=V/2L. For imperfect geometries, a technique uses the discrete sampling of the pulse time domain yet keeps the benefits of the CW phase concepts. This concept, called pulsed phase locked loop (P.sup.2 L.sup.2), permits accurate measurements of equivalent .tau..sub.i -.tau..sub.o times but in the frequency domain.
The disadvantage of pulse time of flight (TOF) measurements stem from two factors. First a pulse is broadband--i.e. contains many frequency components. Many materials are dispersive--the propagation velocity depends on frequency. This fact clearly flaws the TOF concept. When sample attenuation is included, high frequency energy (fast rise time) is lost preferentially to low frequency (slow rise time) energy. Thus, timing errors occur.
Secondly, a TOF measurement requires setting some signal threshold. This of itself produces an error. The threshold crossing time depends then on signal amplitude. Thus both TOF systems are not pure velocity (or time) concepts. The CW technology requires stringent sample geometry to insure plane waves. The P.sup.2 L.sup.2 removes that limitation but can only measure .tau..sub.i -.tau..sub.o equivalent times. What is necessary is a measurement system that is narrowband ("single" frequency), independent of signal amplitude over broad ranges, and able to determine .tau..sub.2 -.tau..sub.i or its equivalent by throwing away .tau..sub.1 -.tau..sub.o initialization.
It is an object of this invention to provide precise measurements of phase shift between two tone burst signals initially derived from the same periodic source but delayed by different amounts through either path length differences, propagation velocity differences, or both.
Another object of this invention is to provide precise measurements of the phase shift of radiation between two surfaces where the radiation is derived from a periodic electrical source.
A further object of this invention is to provide precise measurements of phase shifts independent of an unwanted delay caused by path length effects or propagation effects.
Other objects and advantages of this invention will become apparent hereinafter in the specification and drawings.